1. Field of the Invention
The present invention relates to an imaging apparatus capable of tracking a subject.
2. Description of the Related Art
There is conventionally an imaging apparatus equipped with a so-called subject tracking function which recognizes the position of a main subject in an imaging frame and automatically tracks its position. By using this subject tracking function, it is possible to continuously perform automatic focusing control (AF control) and exposure control with respect to a moving main subject.
As an example of an imaging apparatus equipped with such a subject tracking function, Japanese Patent Application Laid-Open No. 7-110429 discusses an imaging apparatus such as a single-lens reflex camera which has a light metering unit for light metering separate from an imaging element and which tracks a subject based on light metering data output from the light metering unit. However, in the case where subject tracking is performed based on light metering data output from a light metering unit as in the case of the imaging apparatus discussed in Japanese Patent Application Laid-Open No. 7-110429, the following issues arise.
In an ordinary single-lens reflex camera like the imaging apparatus discussed in Japanese Patent Application Laid-Open No. 7-110429, the position of a quick return mirror provided in the camera is moved to switch between a state in which the incident light from the subject is guided to an imaging element and a state in which it is guided to an eyepiece (optical finder).
When the incident light from the subject is guided to the eyepiece, the incident light from the subject is also guided to a light metering unit, making is possible to perform light metering on the subject. In other words, in the state in which the incident light from the subject is guided to the imaging element, light metering cannot be performed at the light metering unit.
More specifically, when the state in which the incident light from the subject is guided to the imaging element and the state in which it is guided to the light metering unit, are successively switched with a short period, as in the case of continuous shooting, the accumulation time of light metering at the light metering unit is limited. In particular, when the light metering data output from the light metering unit is used for subject tracking, it is necessary to complete the subject tracking computation before AF control or exposure control is performed, with the result that the accumulation time is further limited than the period in which the incident light from the subject is guided to the light metering unit.
As a result, the accumulation time of the light metering unit is shorter than the flicker period, which results in an increase in the influence of the flicker, making it unlikely to obtain an accurate light metering value. Here, the flicker will be described. When light metering is performed under a fluorescent lamp or the like, there occurs a so-called flicker phenomenon, in which the brightness of the illumination light periodically varies due to the influence of an AC power frequency. In an area where the power frequency is 50 Hz, the flicker period is approximately 10 ms; in an area where it is 60 Hz, the flicker period is approximately 8.3 ms. When light metering is performed in such an environment, if the accumulation time of the light metering unit is not an integral multiple of the flicker period, a light metering value varies depending on a portion of the phase of the flicker period with which the accumulation period overlaps. Thus, it is difficult to perform exposure control in a stable manner.
If, in view of this, the accumulation time of the light metering unit is lengthened in order to reduce the influence of the flicker, the requisite time until completing the subject tracking computation is also lengthened, resulting in reduction in a shooting speed at the time of continuous shooting.